runtime: improve 386/amd64 systemstack

runtime: improve 386/amd64 systemstack

Minor improvements, noticed while investigating other things.

Shorten the prologue.

Make branch direction better for static branch prediction;
the most common case by far is switching stacks (g==curg).

Change-Id: Ib2211d3efecb60446355cda56194221ccb78057d
Reviewed-on: https://go-review.googlesource.com/97377
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>